Interferon Gamma Release Assays (IGRAs) — a complicated and expensive way to answer a simple question?

Interferon Gamma Release Assays (sometimes ELISPOT tests) are blood tests that can determine whether a person has been exposed to tuberculosis (TB). The test, involves incubating blood samples with proteins unique to TB and then measuring whether T- cells in the blood secrete microbe-fighting gamma interferon —which should occur if someone has previously been exposed to TB.

Two commercial forms of the test, the TB SPOT.TB and Quantiferon-TB Gold tests, have been available in Europe and North America for several years now. IGRAs chief selling points are that they are more sensitive than TST (particularly in people with HIV and suppressed immune responses) and more specific to TB, in other words, they are unlikely to produce false positives because of BCG vaccination (which makes the TST unreliable in children), or because of the boosting effect from serial testing (repeated TSTs can provoke subsequent false positive TST reactions — an important concern when serially testing healthcare workers and other people at high risk of exposure).

However, there is only limited experience using IGRAs in high risk populations and resource-limited settings, and at least one study has found that the increase in sensitivity over TST in people with HIV may not be that great (if a smaller skin reaction is interpreted as a positive result on TST). Furthermore, the tests are too complex to be performed at primary health facilities or the average resource-constrained peripheral laboratory where the results are most needed. And since the blood has to be incubated within hours of being drawn, it would also be very difficult to set up a system where samples are transported to a remote reference laboratory where the test could be performed, at least with the existing test format (Durrant).

According to a recent Médicins Sans Frontieres' report on the TB diagnostics pipeline: “Current versions are not simple, robust diagnostic tests and many technical and medical questions remain associated with use.”

And yet, a number of scientists are exploring whether IGRAs, or a future iteration of these tests, could still revolutionise TB management —if the technology can be used or refined to predict which patients with latent disease will soon develop active TB.

Since over a third of the earth’s population has been exposed to TB, finding a test that will reliably predict active TB is something of a holy grail for TB diagnostics research. Although preventative therapy exists, uptake has been rather low probably because only a relatively low proportion of TB-exposed people go onto develop active disease. A test that could identify those who will could change this.

According to a recent report from FIND and TDR on the Global Demand and market potential for TB diagnostics:

“Worldwide, we estimate that the largest potential available market for a new TB diagnostic would be for a test that both detects latent infection and predicts progression to active disease (767 million patient evaluations/year). Such a test, if widely implemented and accompanied by successful treatment, could revolutionize TB control.”

Since host factors clearly pay the chief role in determining whether a latent TB infection becomes active, a test like IGRA, that specifically measures the immune response to TB, would be the first place to start looking.

“Whether IGRAs have an ability to predict active disease or not is a matter of research and debate,” said Dr Madhukar Pai of McGill University — who has published several literature reviews on the use of IGRAs.

Technically, IGRAs measure the volume of gamma-interferon secreted, and a positive or negative result is determined based upon whether that volume is above or below a certain cut-point. But this yes or no answer could be only scratching the service of what this testing platform could potentially reveal.

Gamma-interferon production appears to be more dynamic than skin test reactivity, and by looking at different cut-off thresholds or changes in this T-cell response it may be possible to identify recent infections, monitor responses to treatment or distinguish between latency and active disease — similar to how changes or different thresholds in CD4 cell counts reflect risk of progression to AIDS.

At present, though, these uses are theoretical and only beginning to be investigated. There are, as of yet, no longitudinal studies demonstrating that IGRA levels or changes in them have any predictive value. Confounding the matter, data from the tests are not consistent across countries. This might be because of lack of experience using the tests (or difficulty using them in some settings), or because the test performs somewhat differently in countries where malnutrition or diseases such as leprosy, helminth infections and HIV are endemic (Dheda). Also little is known about the variability of the interferon-gamma responses within the individual — which is clearly crucial in order to establish if these tests can ever be used in patient management

Several large studies (some sponsored by FIND) that are ongoing or being launched soon should resolve some of these issues and could help define the predictive value of IGRAs as they are currently formatted.

However, it is likely that more basic science research will be needed to modify this lab test. Some ideas being explored include whether the addition of other antigens to the incubation process, or whether short and long periods of incubation affect the results in ways that might reflect differing risks of progression (O’Brien). Its also possible that the measurement of other cytokines (such as IL-10) that might be released by the cells in the blood sample could add to or clarify the predictive value of the tests.

By the time these issues are resolved, the experience from ongoing studies must be applied to simplify the test format to improve the tests’ applicability in low-income, high burden settings.